Mercruiser Alternator Charging Amps?

I?m trying to figure out what size fuses to use on a new ACR dual battery setup. The engine is a 1989 Mercruiser 350Mag. Does anyone know what the charging amps are for this alternator? It?s all stock as far as I can tell.

Re: Mercruiser Alternator Charging Amps?

If it is an I/O it will be over 30 amps and likely 65 amps. A 1989 can be over 100 amps. Most alternators will have a tag or stamp that tells the output.
My 1980 3.0 L 180 cubic inch 140 HP has a 65 amp alternator.
When I bought my boat one of the options was a high output alternator.

Re: Mercruiser Alternator Charging Amps?

Thanks Boatist. I'll see if it has a tag. I don't remember seeing one when I had the engine out a few weeks ago. It seems most replacement alternators are in the 70 amp range. I'll fuse it to that.

Re: Mercruiser Alternator Charging Amps?

Isin't there a curcut breaker on that and not a fuse? I don't have the manual for your year but my 97 shows a CB. and a Mando 55 or 65 amp alt.

Re: Mercruiser Alternator Charging Amps?

Hey rickryder,
I suppose breakers would work. The instructions for installing this blue sea system ACR shows a fuse on each positive lead from the battery. The fuses (or breakers) are sized by the charging power of the alternator. Here's the ACR: http://bluesea.com/productline/docs/387

The instructions say 80-100 amp fuse. I was going to use these: http://bluesea.com/category/5/22/productline/overview/379
They mount on special studs on top of the battery posts. Just to keep it simple.

Re: Mercruiser Alternator Charging Amps?

Do not put a fuse on the alternator output if that is what you are talking about. Wire it straight to the battery or your battery switch.
1989 alternator I believe will be a 3 wire alternator and if put a fuse or breaker in the line and it blows or opens it will destroy and alternator in less than a second. Also if you turn the switch to off and the switch does not have a Alternator Field Disconnect circuit it will blow the alternator.

Wire the alternator straight to the battery so it always has the battery load on the alternator. Then take the +Plus wire off the battery to a large fuse or circuit breaker.

Re: Mercruiser Alternator Charging Amps?

Thanks again for the reply. Just to clarify, here are the fuses I'm refering to.

Re: Mercruiser Alternator Charging Amps?

If your talking about the fuse to the ACR then that is fine.
If that fuse or breaker opens you still have the battery load on the alternator.
All that would happen is you would not be able to charge the second battery if your switch was in the battery 1 position with motor running.

But if you look at the link you attached and look at the motor alternator output it goes to the battery switch common line then trough the switch to battery 1 or 2 directly with no fuse.

It shows an outboard but would be the same for your I/O.
It is very important to never disconnect the battery load from your alternator.

Most battery switches today are what I call a make before break switch.
This means if you turn your battery switch from battery 1 to battery 2 then what actually happens is it connects the second battery before it disconnects the first battery so there is always a load on the alternator.
Most of these will also have a warring to not turn to off with the motor running as this disconnect the batteries load and blows the alternator diode block.

The best switches for older boats will also have an ALTERNATOR FIELD DISCONNECT CIRCUIT. If it is hooked up correctly then it will disconnect the alternator field circuit and stop the alternator from charging before the battery load comes off the alternator. This is to save the alternator diodes.

Cheaper switches do not have the field disconnect circuit so have the warring to not turn off with motor running.

Re: Mercruiser Alternator Charging Amps?

Thanks for the tips Boatist. It's much appreciated. I have a new blue sea switch panel to plug this stuff into, but I'm pretty sure it doesn't have alternater connection. That would be nice though.

For anyone searching, I found that the stock alternater for a 1989 Merc 350Mag is 55 amps per Mercruiser Service Bulletin 91-8.